Electric Power, Alternating Current, Transformers, Magnetic Flux, Turns Ratio, Electric Panel and Circuit Breakers

Posted by PITHOCRATES - February 6th, 2013

Technology 101

AC Power is Superior to DC Power because it can Travel Farther and it Works with Transformers

Thanks to Nikola Tesla and his alternating current electric power we live in the world we have today.  The first electric power was direct current.  The stuff that Thomas Edison gave us.  But it had some serious drawbacks.  You needed a generator for each voltage you used.  The low-voltage of telephone systems would need a generator.  The voltage we used in our homes would need another generator.  And the higher voltages we used in our factories and businesses would need another generator.  Requiring a lot of power cables to hang from power poles along our streets.  Almost enough to block out the sun.

Another drawback is that direct currents travel a long way.  And spend a lot of time moving through wires.  Generating heat.  And dropping some power along the way due to the resistance in the wires.  Greatly minimizing the area a power plant can provide power to.  Requiring many power plants in our cities and suburbs.  Just imagine having three coal-fired power plants around your neighborhood.  The logistics and costs were just prohibitive for a modern electric world.  Which is why Thomas Edison lost the War of Currents to Nikola Tesla.

So why is alternating current (AC) superior to direct current (DC) for electric power?  AC is more like a reciprocating motion in an internal combustion engine or a steam locomotive.  Where short up & down and back & forth motion is converted into rotation motion.  Alternating current travels short distances back and forth in the power cables.  Because they travel shorter distances in the wires they lose less power in power transmission.  In fact, AC power lines can travel great distances.  Allowing power plants tucked away in the middle of nowhere power large geographic areas.  But there is another thing that makes AC power superior to DC power.  Transformers.

The Voltage induced onto the Secondary Windings is the Primary Voltage multiplied by the Turns Ratio

When an alternating current flows through a coiled wire it produces an alternating magnetic flux.  Magnetic flux is a measure of the strength and concentration of the magnetic field created by that current.  When this flux passes through another coiled wire it induces a voltage on that coil.  This is a transformer.  A primary and secondary winding where an alternating current applied on the primary winding induces a voltage on the secondary winding.  Allowing you to step up or step down a voltage.  Allowing one generator to produce one voltage.  While transformers throughout the power distribution network can produce the many voltages needed for doorbells, electrical outlets in our homes and the equipment in our factories and businesses.  And any other voltage for any other need.

We accomplish this remarkable feat by varying the number of turns in the windings.  If the number of turns is equal in the primary and the secondary windings then so is the voltage.  If the number of turns in the primary windings is greater than the number of turns in the secondary windings the transformer steps down the voltage.  If the number of turns in the secondary windings is greater than the number of turns in the primary windings the transformer steps up the voltage.  To determine the voltage induced onto the secondary windings we divide the secondary turns by the primary turns.  Giving us the turns ratio.  Multiplying the turns ratio by the voltage applied to the primary windings gives us the voltage on the secondary windings.  (Approximately.  There are some losses.  But for the sake of discussion assume ideal conditions.)

If the turns ratio is 20:1 it means the number of turns on the primary windings is twenty times the turns on the secondary windings.  Which means the voltage on the primary windings will be twenty times the voltage on the secondary windings.  Making this a step-down transformer.  So if you connected 4800 volts to the primary windings the voltage across the secondary windings will be 240 volts (4800/20).  If you attached a wire to the center of the secondary coil you can get both a 20:1 turns ratio and a 40:1 turns ratio.  If you measure a voltage across the entire secondary windings you will get 240 volts.  If you measure from the center of the secondary and either end of the secondary windings you will get 120 volts.

The Power Lines running to your House are Two Insulated Phase Conductors and a Bare Neutral Conductor

This is a common transformer you’ll see atop a pole in your backyard.  Where it is common to have 4800-volt power lines running at the top of poles running between houses.  On some of these poles you will see a transformer mounted below these 4800-volt lines.  The primary windings of these transformers connect to the 4800-volt lines.  And three wires from the secondary windings connect to wires running across these poles below the transformers.  Two of these wires (phase conductors) connect to either end of the secondary windings.  Providing 240 volts.  The third wire attaches to the center of the secondary windings (the neutral conductor).  We get 120 volts between a phase conductor and the neutral conductor.

The power lines running to your house are three conductors twisted together in a triplex cable.  Two insulated phase conductors.  And a bare neutral conductor.  These enter your house and terminate in an electric panel.  The two phase conductors connect to two bus bars inside the panel.  The neutral conductor connects to a neutral bus inside the panel.  Each bus feeds circuit breaker positions on both sides of the panel.  The circuit breaker positions going down the left side of the panel alternate between the two buss bars.  Ditto for the circuit breaker positions on the right side.

A single-pole circuit breaker attaches to one of the bus bars.  Then a wire from the circuit breaker and a wire from the neutral bus leave the panel and terminate at an electrical load.  Providing 120 volts to things like wall receptacles where you plug things into.  And your lighting.  A 2-pole circuit breaker attaches to both bus bars.  Then two wires from the circuit breaker leave the panel and attach to an electrical load.  Providing 240 volts to things like an electric stove or an air conditioner.  Then a reciprocating (push-pull) alternating current runs through these electric loads.  Driven by the push-pull between the two bus bars.  And between a bus bar and the neutral bus.  Which is driven by the push-pull between the conductors of the triplex cable.  Driven by the push pull of secondary windings in the transformer.  Driven by the push-pull of the primary windings.  Driven by the push-pull in the primary cables connected to the primary windings.  And all the way back to the push-pull of the electric generator.  All made possible thanks to Nikola Tesla.  And his alternating current electric power.

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Electric Grid, Voltage, Current, Power, Phase Conductor, Neutral Conductor, 3-Phase Power, Transmission Towers and Corona Discharge

Posted by PITHOCRATES - August 15th, 2012

Technology 101

The Electric Grid is the Highways and Byways for Electric Power from the Power Plant to our Homes

Even our gasoline-powered cars operate on electricity.  The very thing that ignites the air-fuel mixture is an electric spark.  Pushed across an air-gap by a high voltage.  Because that’s something that high voltages do.  Push electrons with such great force that they can actually leave a conductor and travel through the air to another conductor.  Something we don’t want to happen most of the time.  Unless it’s in a spark plug in our gasoline engine.  Or in some movie prop in a cheap science fiction movie.

No.  When we use high voltage to push electrons through a conductor the last thing we want to happen is for the electrons to leave that conductor.  Because we spend a pretty penny to push those electrons out of a power plant.  And if we push the electrons out of the conductor they won’t do much work for us.  Which is the whole point of putting electricity into the electric grid.  To do work for us.

The electric grid.  What exactly is it?  The highways and byways for electric power.  Power plants produce electric power.  And send it to our homes.  As well as our businesses.  Power is the product of voltage and current.  In our homes something we plug into a 120V outlet that draws 8 amps of current consumes 960 watts.  Which is pretty big for a house.  But negligible for a power plant generator producing current at 20,000 volts.  For at 20,000 volts a generator only has to produce 0.48 amps (20,000 X 0.48 = 960).  Or about 6% of the current at 120V.

Between our Homes and the Power Plant we can Change that Current by Changing the Voltage

Current is money.  Just as time is money.  In fact current used over time helps to determine your electric bill.  Where the utility charges you for kilowatt hours (voltage X current X time).  (This would actually give you watt-hours.  You need to divide by 1000 to get kilowatt hours.)  The electric service to your house is a constant voltage.  So it’s the amount of current you use that determines your electric bill.  The more current you use the greater the power you use.  Because in the power equation (voltage X current) voltage is constant while current increases.

Current travels in conductors.  The size of the conductor determines a lot of costs.  Think of automobile traffic.  Areas that have high traffic volumes between them may have a very expensive 8-lane Interstate expressway interconnecting them.  Whereas a lone farmer living in the ‘middle of nowhere’ may only have a much less expensive dirt road leading to his or her home.  And so it is with the electric grid.  Large consumers of electric power need an Interstate expressway.  To move a lot of current.  Which is what actually spins our electrical meters.  Current.  However, between our homes and the power plant we can change that current.  By changing the voltage.  Thereby reducing the cost of that electric power Interstate expressway.

The current flowing through our electric grid is an alternating current.  It leaves the power plant.  Travels in the conductors for about 1/120 of a second.  Then reverses direction and heads back to the power plant.  And reverses again in another 1/120 of a second.  One complete cycle (travel in both directions) takes 1/60 of a second.  And there are 60 of these complete cycles per second.  Hence the alternating current.  If you’re wondering how this back and forth motion in a wire can do any work just think of a steam locomotive.  Or a gasoline engine.  Where a reciprocating (back and forth) motion is converted into rotational motion that can drive a steam locomotive.  Or an automobile.

The Voltages of our Electric Grid balance the Cost Savings (Smaller Wires) with the Higher Costs (Larger Towers)

An electric circuit needs two conductors.  When current is flowing away from the power plant in one it is flowing back to the power plant in the other.  As the current changes direction is has to stop first.  And when it stops flowing the current is zero.  Using the power formula this means there are zero watts twice a cycle.  Or 120 times a second.  Which isn’t very efficient.  However, if you bring two other sets of conductors to the work load and time the current in them properly you can remove these zero-power moments.  You send the first current out in one set of conductors and wait 1/3 of a cycle.  Then you send the second current out in the second set of conductors and wait another 1/3 cycle.  Then you send the third current out in the third set of conductors.  Which guarantees that when a current is slowing to stop to reverse direction there are other currents moving faster towards their peak currents in the other conductors.  Making 3-phase power more efficient than single-phase power.  And the choice for all large consumers of electric power.

Anyone who has ever done any electrical wiring in their home knows you can share neutral conductors.  Meaning more than one circuit coming from your electrical panel can share the return path back to the panel.  If you’ve ever been shocked while working on a circuit you switched off in your panel you have a shared neutral conductor.  Even though you switched off the circuit you were working on another circuit sharing that neutral was still switched on and placing a current on that shared neutral.  Which is what shocked you.  So if we can share neutral conductors we don’t need a total of 6 conductors as noted above.  We only need 4.  Because each circuit leaving the power plant (i.e., phase conductor) can share a common neutral conductor on its way back to the power plant.  But the interesting thing about 3-phase power is that you don’t even need this neutral conductor.  Because in a balanced 3-phase circuit (equal current per phase) there is no current in this neutral conductor.  So it’s not needed as all the back and forth current movement happens in the phase conductors.

Electric power travels in feeders that include three conductors per feeder.  If you look at overhead power lines you will notice they all come in sets of threes when they get upstream of the final transformer that feeds your house.  The lines running along your backyard will have three conductors across the top of the poles.  As they move back to the power plant they pass through additional transformers that increase their voltage (and reduce their current).  And the electric transmission towers get bigger.  With some having two sets of 3-conductor feeders.  The higher the voltage the higher off the ground they have to be.  And the farther apart the phase conductors have to be so the high voltage doesn’t cause an arc to jump the ‘air gap’ between phase conductors.  As you move further away from your home back towards the power plant the voltage will step up to values like 2.4kV (or 2,400 volts), 4.8kV and13.2kV that will typically take you back to a substation.  And then from these substations the big power lines head back towards the power plant.  On even bigger towers.  At voltages of 115kV, 138kV, 230kV, 345kv, 500kV and as high as 765kV.  When they approach the power plant they step down the voltage to match the voltage produced by its generators.

They select the voltages of our electric grid to balance the cost savings (smaller wires) with the higher costs (larger towers taking up more land).  If they increase the voltage so high that they can use very thin and inexpensive conductors the towers required to transmit that voltage safely may be so costly that they exceed the cost savings of the thinner conductors.  So there is an economic limit on voltage levels  As well as other considerations of very high voltages (such as corona discharge where high voltages create such a power magnetic field around the conductors that it may ionize the air around it causing a sizzling sound and a fuzzy blue glow around the cable.  Not to mention causing radio interference.  As well as creating some smog-causing pollutants like ozone and nitrogen oxides.)

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